Abstract
We report on strong backward stimulated emission at 337 nm in nitrogen gas pumped by circularly polarized femtosecond laser pulses at 800 nm. A distinct dependence of the backward UV spectrum on pump laser polarization and intensity is observed, pointing to the occurrence of backward amplified spontaneous emission inside filaments. We attribute the population inversion to inelastic collision between the free electrons produced by the pump laser and neutral N2 molecules. The addition of oxygen molecules is detrimental for the gain, reducing it to near threshold at atmospheric concentration.
Highlights
Ambient air pumped by intense ultrashort laser pulses can give birth to stimulated radiation in both the forward and backward directions [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
2andre.mysyrowicz@ensta-paristech.fr * yi.liu@ensta-paristech.fr; Abstract: We report on strong backward stimulated emission at 337 nm in Nitrogen gas pumped by circularly polarized femtosecond laser pulses at 800 nm
We believe that this simple scheme for backward stimulated emission from Nitrogen gas pumped by the widely available 800 nm femtosecond laser pulse is a significant step towards applications for remote sensing
Summary
Ambient air pumped by intense ultrashort laser pulses can give birth to stimulated radiation in both the forward and backward directions [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The backward stimulated radiation (propagating in the direction opposite to the pump laser) is of particular interest for remote sensing, and has attracted considerable attention [1,2,3,4,5,6,7,8]. The advantage of the backward stimulated radiation for remote sensing lies in the fact that it can bring information about pollutants towards the ground observer with a well defined directionality. This has to be compared to the omni-directionality of the fluorescence or scattered optical signal from the same pollutants.
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